Electron beam generating system with collimated focusing means

ABSTRACT

A storage system for the mass recording and readout of digital data with ultra high resolution. An electron beam structure is provided for forming a beam of extremely small focused spot diameter, on the order of 0.1 microns, and high current density capability, on the order of 1,000 amperes per sq. cm., which records data by scanning over defined areas of the storage medium surface and micromachining elemental portions of said medium as a function of beam modulation. Readout may be subsequently accomplished by similarly scanning the beam at reduced power density and detecting electrons that have been transmitted by or reflected from the storage medium.

United States Patent [191 Wolfe et al.

[ Nov. 5, 1974 1 ELECTRON BEAM GENERATING SYSTEM WITH COLLIMATEDFQCUSLNG. MEANS.

[75] lnventors: John E. Wolfe, Camillus; George E.

Ledges, Liverpool; Homer H. Glascock, Scotia, all of NY.

[73] Assignee: General Electric Company,

Syracuse, NY.

[22] Filed: Aug. 6, 1969 [21] Appl. No.: 847,972

[52] US. Cl. ..313/421, 315/31,

[56] References Cited UNITED STATES PATENTS 8/1944 Ruska 313/84 11/1966Day 313/74 X 3,374,386 3/1968 Charbonnier et a1 313/336 X PrimaryExaminerRobert Segal Attorney, Agent, or Firm-Richard V. Lang; Carl W.Baker; Frank L. Neuhauser [5 ABSTRACT A storage system for the massrecording and readout of digital data with ultra high resolution. Anelectron beam structure is provided for forming a beam of extremelysmall focused spot diameter, on the order of 0.1 microns, and highcurrent density capability, on the order of 1,000 amperes per sq. cm.,which records data by scanning over defined areas of the storage mediumsurface and micromachining elemental portions of said medium as afunction of beam modulation, Readout may be subsequently accomplished bysimilarly scanning the beam at reduced power density and detectingelectrons that have been transmitted by or reflected from the storagemedium.

6 Claims, 11 Drawing Figures PATENTEDNUY 51974 SHEET 305 3 PATENTEDNnv51914 $846660 LOGIC NETWORK INVENTORS',

AMPERES/CM l0" JOHN E. WOLFE,

GEORGE E. LEDGES, HOMER H. GLASCOCK I04 I65 ld ld lo: THEIR ATTORNEY.

LEC B NER .sxsrau .wna.

COLLIMATED FOCUSING MEANS BACKGROUND OF THE INVENTION 1. Field of theInvention The invention relates to the field of mass storage andretrieval systems for storing huge quantities of high resolution dataand, in particular, to high density systems of this type employingelectron beam recording and readout.

2. Description of the Prior Art A figure of merit of data bits capacityhas evolved for mass storage and retrieval systems. A capacity of thismagnitude is considered desirable in order to satisfy present dayarchival storage requirements and workers in the field continue to beengaged in finding the most effective means for its accomplishmentwithin a confined space. The principal limitation of storage systemsdeveloped to date is the resolution of the storage data. Lacking veryhigh resolution, these systems require extensive storage space forstoring large quantities of data and access times to said data arenecessarily slow. In order to index elemental data bits for eitherstorage or retrieval purposes, a relatively complex and slow mechanicalaccessing operation is normally required. In one system known to the arta laser beam is employed for recording and reading out data on aphotosensitive storage medium which is in the form of a long continuoustape. Information is written by scanning the beam diagonally across thetape as the tape moves longitudinally, providing parallel scan linesthroughout the tapes length. Because of inherent limitations in thewavelength of laser energy, on the order or 5,000 to 6,000 angstroms,data can be recorded with resolutions no greater than a few microns.Thus, for a 10 bit memory, 2,400 feet of 8 mm tape must be provided. Itmay be appreciated that lengthy access times are required for thissystem.

Another existing approach is to employ an electron beam for writing onphotographic film. This has been accomplished employing a beam having athree micron resolution and recording on 35 mm chips. For a 10 bitstorage capability there are required 200,000 individual chips. Themechanical accessing requirements for this system are extremely complex.Further, it is necessary to develop the photographic film before theinformation can be read out or checked, and data cannot be subsequentlyentered.

In the field of electron microscopy it has been suggested to employ ascanned electron beam of extremely small spot diameter, such aspresently utilized in scanning electron microscopes, to record digitalinforma tion by means of selective etching or a comparable technique.The art has been developed to where there presently exist beam formingapparatus which generate beam spot diameters on the order of severalhundred angstroms and less, corresponding to a resolution orders ofmagnitude higher than in the above noted systems. However, these are allrelatively low power density beams, not capable of providing directlypermanent data storage using presently available recording material s.Accordingly, there does not exist at the present time apparatus forgenerating electron beams of minute dimensions for an ultra highresolution recording which also have extremely high current densitycharacteristics for a permanent data storage.

SUMMARY OF THE INVENTION It is accordingly a principal object of thisinvention to provide a novel ultra high density storage sytem whichpermanently and directly records data at appreciably higher resolutionsthan presently obtainable, making possible the rapid storage of hugequantities of data within a confined space.

It is a further object of the invention to provide a novelstorage-system as above described wherein recording and readoutoperations are accomplished at high speed, and do not require a separatedeveloping step.

A further object of the invention is to provide a novel storage systemas described wherein exceedingly large quantities of data, on the orderof 10 bits and greater, can be stored on a single, relatively small,storage surface.

It is still another object of the invention to provide a novel ultrahigh density storage system as above described in which data is recordedby micromachining elemental portions of the storage medium by means of ascanned electron beam, the stored data being read out also by a scannedelectron beam.

A still further object of the invention is to provide a novel electronbeam forming structure for generating a beam of extremely small spotdiameter, on the order of 0.1 microns, and high current density, on theorder of 1,000 amperes per sq. cm.

A yet further object of the invention is to provide a novel electronbeam forming structure for generating a beam with the above notedcharacteristics by means of a relatively low accelerating voltage, onthe order of 5 to 10 KV.

It is another object of the invention to provide a novel electronemission system having a field aided thermionic cathode that emitselectrons with an exceedingly high current density and is extremelystable in its operation.

Still another object of the invention is to provide a novel field aidedthermionic cathode employing a tungsten needle coated with an atomiclayer of zirconium which exhibits an extremely long lifetime, on theorder of 1,000 hours and greater.

It is yet a further object of the invention to provide a novel electronoptical system for forming a high current density, small spot diameterbeam that is capable of being deflected over a relatively large numberof resolvable elements.

Still a further object of the invention is to provide a novel electronoptical system as above described in which the effective sphericalaberration of the focus lenses is made low for relatively large focallengths.

In accordance with these and other objects of the invention there isprovided an ultra high density storage system which employs a scannedelectron beam of extremely small spot size and high current density torecord data on a storage medium by micromachining elemental portions ofsaid medium. Readout of the stored data is accomplished by means of thescanned electron beam, modulated electrons from the target storagemedium being collected by a detector device. The readout electron beamis at reduced current densities which will not destroy the stored databits. For high capacity storage, data is stored as numerous discretedata blocks over each of which the electron beams can be magnetically orelectrically scanned. Mechanical drive means are provided for indexingthe data blocks with respect to said beam for both write and readoutoperations.

With respect to one specific aspect of the invention, the electron beamis produced by a novel electron emission system through a process offield aided thermionic emission. The electron emission system iscomposed of a cathode including a filamentary hairpin with a weldedsingle crystal oriented tungsten needle which is of extremely smalldimension at the emissive cathode tip. Sintered zirconium is applied ina ball at the base of the needle, and upon heating of the hairpin andneedle migrates as a solid up the needle to the tip. The zirconiumcoating acts to reduce the work function of the 100 face on the tip ofthe tungsten crystal to a value appreciably lower than occurring onother faces. The emission system further includes an apertured anode andgrid electrode structure for generating a spherical electric fieldconfiguration about the emissive cathode tip which exhibits a very highfield gradient at said tip for causing a high power density electronemission.

With respect to a second specific aspect of the invention, an electronoptical system is provided which includes first and second focus lensesto provide a single stage imaging of the electrons emitted from thecathode tip onto the target. The cathode tip, which is at about theobject plane, is positioned at the focal point of the first lens and thetarget, which is at the image plane, is positioned at the focal point ofthe second lens. The focused beam impinges on the target at sufficientlyhigh power densities to vaporize away portions of the target material.Modulation of the beam is accomplished by a modulation coil which shiftsthe beam axis with respect to a limiting aperture provided in thevicinity of the beam source. A set of deflection coils are providedforward of the final focusing lens for deflecting the beam in both X andY directions in the plane of the storage medium.

BRIEF DESCRIPTION OF THE DRAWING The specification concludes with claimsparticularly pointing out and distinctly claiming the subject matterwhich is regarded as the invention. It is believed, however, that bothas to its organization and method of operation, together with furtherobjects and advantages thereof. the invention may be best understoodfrom the description of of the preferred embodiments, taken inconnection with the accompanying drawings in which:

FIG. 1 is a schematic diagram in a partially broken away perspectiveview of an electron beam ultra high density storage system in accordancewith one embodiment of the invention employing a limited area storagemedium;

FIG. 2 is an enlarged side view of the storage medium and electrondetector employed in the system of FIG. 1;

FIG. 3 is a partial plan view of the storage medium employed in thesystem of FIG. 1, illustrating the written data format;

FIG. 4 is a series of graphs illustrating the formation of the inputmodulation signal;

FIG. 5 is a side view of a modified reflective readout structure;

FIG. 6 is a detailed cross sectional view of the total electron beamstructure of FIG. 1;

FIG. 7 is an enlarged cross sectional view of the electron emissionstructure of FIG. 6;

FIG. 8 is a further enlarged cross sectional view of the cathodestructure;

FIG. 9 are several curves illustrating field aided thermionic emission;

FIG. 10 is a schematic diagram in partially broken away perspective viewof a further embodiment of the invention employing a large area storagemedium and mechanical drive means for indexing said storage medium; and

FIG. 11 is a partial plan view of the storage medium employed in thesystem of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there isschematically illustrated in perspective view an electron beam storagesystem for permanently storing data at ultra high densities, exceeding10 data bits per sq. cm. The data is written by a scanned electron beamof extremely small beam spot diameter, on the order of 0.1 micron, andextremely high current density. A magnetic deflection was employed inthe illustrated system, although an electrostatic deflection might alsobe used. The writing beam, at the focused spot, exhibits a currentdensity of IO amperes per sq. cm. and greater, and therefore a powerdensity of 10 watts per sq. cm. and greater within moderate anodevoltages. The writing beam is modulated as a function of the input dataso as to selectively micromachine by vaporization elemental portions ofthe medium 1 as the beam is scanned over its surface. Scan rates as highas 10 data bits per second and higher are employed. A non-destructivereadout of the stored data is accomplished by a scanned readout electronbeam operatedat reduced power l evels about one-tenth that of thewriting beam.

The electron beam structure for both record and readout includes anelectron emission system 3 and an electron optical system 4 associatedwith an evacuated chamber 5 within which the beam is enclosed. Theemission system 3 produces the electron beam and includes among itsprincipal components a cathode structure 7, grid electrode 8 and anodeelectrode 9. The electron optical system 4 controls, focuses anddeflects the beam and principally includes focusing coils 10A and 10B,deflection coils 11 and modulation coils 12. The emission system 3 andelectron optical system 4 include features of novelty that make possiblethe extremely high resolution, high current density properties of thefocused electron beam, and will be described in greater detail whenconsidering FIGS. 6 and 7. An input means 13, shown in block form,supplies input signals for modulating the beam. Input means 13 maycomprise a conventional source of digital data, e.g., the peripheralequipment of a digital computer. Although, in the disclosed embodimentsof the invention the modulation signal contains digital data, theinvention need not be so limited and may be useful with other data formssuch as analog and alphanumeric data.

A medium vacuum, on the order of 10' mm. Hg, is provided within thechamber 5 by a vacuum pump 14, schematically illustrated in block form.A vacuum of this magnitude represents a compromise that it is relativelyeasy to achieve and maintain, while being compatible with the high orderof performance required of the present system. Also housed within thechamber 5 is the readout structure in the form of an electron detector15 upon which the storage medium 1 is deposited. The storage medium 1and detector are mounted on a base structure 16. An output means 17 isconnected to the detector 15 for receiving the readout data. A mastercontrol logic network 18 is coupled to input means 13 and output means17. The network 18 includes numerous logic circuits of standard designfor providing the sundry logic functions which control the writing andreadout operations, as well as a clock frequency generator for supplyinga master timing of said operations.

The electron detector 15 may be a conventional component. In theembodiment being considered it is a single crystal silicon p-i-njunction device which in response to penetrating electrons of thereadout beam generates electron-hole pairs. As shown in the side view ofFIG. 2, the silicon detector includes a p region 20, an intrinsic region21 and an n region 22, with contacts 23 and 24 made to the p and nregions, respectively. The storage medium 1 is deposited as a thin filmover the p region 20. A dc. voltage source 25 is connected through alead resistor 26 to contacts 23 and 24 for reverse biasing the device.Output terminals 27 are coupled through a capacitor 28 to contacts 23and 24 for sensing readout current flow through the device 15.

During the write operation the electron beam is operated at high currentand power densities. As it scans across the surface of the storagemedium 1, the beam selectively micromachines elemental portions thereof,corresponding to the data bits, as a function of beam modulation. Aheating and rapid vaporization of said elemental portions of thematerial occur in response to penetration by the beam's high velocityelectrons. With respect to the vaporization process, the high densityelectrons of the focused beam spot penetrate the storage material withrelatively high kinetic energy. As the electrons are rapidly deceleratedby the bulk of the material, heat is given off which in a localized areaelevates the temperature to a point well above the threshold temperatureof the vapor pressure versus temperature curve of the material, thethreshold temperature being that temperature at which the vapor pressurecommences to rise steeply as a function of temperature. At this elevatedtemperature, the vapor pressure of said localized area is raised ordersof magnitude above the ambient pressure and rapid vaporization of thematerial results.

It is preferred to modulate the beam by applying the input signal to themodulation coils 12 for recurrently shifting the beam from its centralaxis soas to be partially intercepted during its travel, therebymaintaining a constant current emission while modulating the currentdensity at the focused spot. The beam is rapidly scanned by thedeflection coils 11 along parallel data tracks successively formed onthe surface of the storage medium.

In the embodiment of FIG. 1, the storage medium 1 has a storage areaabout 36 mils square with about 4,500 data lines and about 4,500resolvable elements per data line. Thus, there is provided a storagecapacity in excess of 2 X 10 bits. With reference to FIG. 3', a smallarea of the storage medium 1 in a greatly magnified plan view isillustrated, showing four data strips 31, 3'2, 33 and 34, the edges ofwhich comprise the data lines. Tracks 35, formed between the datastrips, are employed: to servo the readout beam as will be furtherexplained. Information is written along both edges of each data strip atresolvable elements on the data lines, such as shown with respect toresolvable elements m and n. In accordance with standard practice, thedata lines have a string of known data bits written at the beginning ofeach line which are used as a reference during readout. In the presentformat the data strips are on centers spaced apart by .4 microns, andeach resolvable element is 0.2 microns in length. Digital information ofbinary 1s and "0s is written as a 180 phase modulation of a square waveat one-half the reference clock frequency supplied by the logic network18, so as to correspondingly micromachine the data strips during eitherthe first or second half of travel of the beam through each resolvableelement. The writing beam may vaporize essentially the entire thicknessof the storage medium or only a fraction of this thickness.

Referring to the diagram of FIG. 4, the binary bits may be fed in as aseries of ls and 0s at two corresponding d.c. levels, as shown by theGraph A. It will be assumed that the bits are supplied at a 10 MHz rate.The clock signal, shown in Graph B, is at twice the data rate or 20 MHZ.Graph C illustrates the phase modulated square wave corresponding to thedata bits in Graph A that is employed as the input signal for modulatingthe beam. With clock signal at a 20 MHz rate, a single data bit iswritten in 0.1 microseconds.

For the operation under consideration, the storage medium 1 must be astable material capable of being selectively and rapidly vaporized atthe requisite resolution of the system by the writing electron beam, andyet be totally unaffected by the lower energy readout beam. Principalproperties of the medium 1 include a relatively high vapor pressure atthe writing temperature, and a vapor pressure that is a steep functionof temperature above threshold; a high density; and a low thermalconductivity. A high vapor pressure of the writing temperature causesthe material to rapidly vaporize in response to heating by the writingbeam. The steep vapor pressure versus temperature function permits areduced power readout operation that can produce no vaporization of thematerial. High density and low thermal conductivity properties permit alocalized heating of the material necessary for high resolution writing.The assignment of specific values for the above noted properties isdependent upon the writing and readout beam parameters as well as systemrequirements for resolution, speed, etc., and the interrelationship ofthese factors. For example, the requirements for high vapor pressure,high density and low thermal conductivity are inversely related to thewriting beam current density.

In accordance with existing performance specifications for a givensystem, the storage medium may be selected from various classes ofmaterials including semimetallic, semiconductor and dielectricmaterials. In the embodiment under consideration there is employed analloy of selenium with 10-20 percent arsenic for retaining an amorphousform of the selenium. This material has a specific gravity of about 4.3,a thermal conductivity of about 10 calories per sec. per cmC, and avapor pressure of about l0 mm. Hg. at a writing temperature of 700C,which pressure reduces to about 10 mm. Hg. at a readout temperature ofC. The material is deposited on one surface of the electron detector 8as a thin film, having a thickness of approximately 1,500 to 3,000 A.

Considering the readout operation, the electron beam is operated atreduced power density. This may be accomplished by several differentmeans, but it is preferable to reduce the current density of the focusedspot through partial interception of the beam by biasing the beam offits central axis. The reduced power density causes a correspondinglyreduced heating of the storage medium, and a very greatly reduced vaporpressure, as previously noted. In the embodiment of FIG. 1, as thereadout beam is scanned the electrons from the beam are transmittedthrough the etched portions of the data elements and penetrate theelectron detector 15. Electron-hole pairs are created by the penetratingelectrons which generate a corresponding readout signal from outputmeans 17. The readout signal contains the readout data in the form ofphase information similar to the input signal. To maintain accuracy ofthe readout signal, this signal is synchronized with the clock frequencyduring the readout of each data bit.

In order that the readout beam precisely follow each data line, a servosystem is provided for sensing and correcting any tendency for beamoffset. One of several conventional servo techniques may be employed. Inthe storage system of FIG. 1, an edge servo system is used wherein asthe beam is scanned along a data line, displacement from the edge of thedata track is sensed and a correction signal generated. A servo unit,including a low pass filter and an error sensor to which the readoutsignal is coupled, may be embodied within the output means 17. As thebeam travel may tend to deviate from a scanned data line toward or awayfrom the adjoining servo track, a low frequency component is introducedinto the readout signal the magnitude of which is a function of the beamdisplacement. In response to said low frequency component, the servounit generates a correction signal which is coupled to the verticaldeflection coils for compensating the beams travel.

In FIG. is illustrated an alternate embodiment of the readout structureof the reflective type. In this embodiment, a storage medium 41 isdeposited upon a supporting substrate 42, e.g., of glass. An electrondetector 43 is positioned above the storage surface and offset from theimpinging electron beam. The electron detector 43, which may be one ofseveral conventional types including the p-i-n junction deviceillustrated in FIG. 2, receives readout electrons which are reflectedfrom the storage medium surface. The back scattered electrons can bereflected primary or secondary electrons, or both. An acceleratingpotential, not shown, is applied in known fashion to the detector forsensing secondary electrons. The readout operation is otherwise similarto that previously described, the detector responding to the reflectedelectrons for generating electron-hole pairs within its volume, which inturn provides a corresponding readout signal. Other forms of electrondetectors such as channel multipliers and photon devices may also beemployed.

A cross sectional view of the total electron beam structure includingthe electron emission system 3 and the electron optical system 4 isshown in FIG. 6, taken along the plane 6-6 in FIG. I. An enlarged crosssectional view of the electron emission system per se is illustrated inFIG. 7, and a further enlarged view of the cathode structure is shown inFIG. 8. The electron beam structure of FIGS. 6 and 7 forms an electronbeam having a theoretical current density j at the focused spot on thetarget that may be defined by Langmuirs equation as follows:

j=j (l+(eV/KT)) sin a where j is the emission current density at thecathode emissive surface;

e is the electron charge;

V is the voltage at the target;

K is Boltzmans constant;

T is the absolute temperature; and

a is the half angle at the focused spot.

With reference to the above equation, it may be appreciated that therequirements of the storage system impose a number of significantconstraints on the electron beam structure in providing extremely hightarget current densities. Thus, a prime consideration for obtaining ahigh target current density j is to maximize the cathode emissioncurrent density j,,. The current density j is also proportional to thetarget voltage V. However, the voltage V also determines the velocity atwhich the electrons strike the target and an excessively high voltagewill result in expanding the elemental heated portions of the storagemedium and degrading resolution. Thus, the value of V must be determinedwith these conflicting considerations in mind. From the above equationit is also seen that the current density j is inversely proportional tothe temperature T, which places a limitation on heating of the cathode.

Referring to FIG. 7, the cathode structure 7 includes a hairpin filament50 having a cathode needle 51 joined at the vertex of said hairpin. Apotentiometer, including a dc. source 52 in shunt with a resistor 53, iscoupled to the terminals of the filament 50 for heating said filament. Anegative high voltage source -V, is coupled to a tap on the resistor 53.A shield 54 surrounds the cathode, grid and a part of the anodestructure. The grid electrode 8 is in the form of a disk having anaperture 55 through which the cathode needle extends. A negative voltagesource -V is coupled to the grid 8, where V is slightly more negativethan V,. The anode electrode 9 is of the re-entrant type, the reentrantportion being provided with a central aperture 56 positioned immediatelyforward of the cathode tip. The anode electrode 9 is at groundpotential, as is all structure forward of anode. At the opposite orforward end of the anode electrode is a limiting apertured electrode 57in the shape of a disk having a central limiting aperture 58. Acylindrical sleeve 59 encloses the described emission structure. Severalpassages in the grid and anode structure, such as at 67, 68 and 69,facilitate evacuation of the electron emission region. The anodeelectrode, grid electrode and cathode needle structure together with thepotentials applied thereto produce a hemispherical electric fieldconfiguration around the cathode tip, with the tip at the radial centerof the hemisphere. The hemispherically configured electric field incombination with the extremely small dimensions of the cathode tipproduce a very high electric field gradient in the vicinity of said tip.The hemispherical field also limits aberrations in the focused beam.

In one operable structure, in accordance with the invention, the cathodeneedle 51 is about 30 mils in length and extends forward of the gridelectrode 8 by about 10 mils. This is the dimension g in FIG. 8. Thegrid electrode shields the hairpin and assists in limiting emission tothe tip of the cathode needle, as well as in shaping the hemisphericalelectric field. The emissive surface at the cathode needle tip has aradius of about 1 micron. The grid electrode aperture 55 is about 10mils in diameter. The anode electrode 9 is about 30 mils forward of thegrid electrode 8, shown as the dimension h in FIG. 8. The anodeelectrode is about 1.125 in. wide at the forward end and has a totallength in the axial direction of about 1.1 in. The anode aperture 56 isabout 10 mils in diameter and the limiting aperture 58 is about 20 milsin diameter. For the indicated length, dimension and forward extensionof the cathode needle 51, the grid to anode spacing and the dimensionsof the grid, anode and limiting apertures, the voltage V, was at 5.0 KVand the voltage V-,; at 5.3 KV. An electric field gradient of 10 V/cm,was thereby provided at the cathode tip. For a voltage V, of 10.0 KV andV of 10.3 KV, the grid to anode spacing is modified, computed to beabout 40 mils, for retaining the 10 V/cm. electric field gradient at thecathode tip. The filament was heated to a temperature of approximately1,800K. This temperature keeps the cathode tip clean of contaminatingadsorbed atoms in the medium vacuum that is used. The extremely highelectric field gradient in combination with heating of the filament 50produces a high density field aided thermionic emission from the cathodetip. It is noted that the high field gradient of 10" V/cm. is obtainedwith a moderate anode voltage of less than 5 KV to about KV. Thesevalues of voltage, particularly at the lower end, are found not toprovide an excessively great velocity of electrons striking the presenttarget which might cause diffuse heating of the target such as todegrade resolution. In addition, for extremely thin targets, overly highvelocity electrons may penetrate completely through and not generatesufficient heat in the target material.

In accordance with the operable embodiment under consideration, thehairpin filament 50 is composed of rhenium selected for its refractoryand ductile characteristics. The filament has a diameter of about 10mils reduced to 7 mils at the vertex, as indicated in the enlargeddrawing of FIG. 8. The cathode needle 51 is an oriented single crystaltungsten having the 100 crystal face at the needle tip, which is thepreferential face for lowering the work function. The 100 crystal faceis orthogonally related to the needle longitudinal axis within a 1degree limit, preferably. The needle 51 is welded to the filament 50. Aslurry of zirconium hydride is applied as a bead to the base of theneedle 51 around the weld point. Upon heating of the filament, thezirconium hydride becomes sintered to form zirconium. The zirconiummigrates over the surface of the needle and covers the tip, providingcontinuous replenishment for the effects of evaporation and ionbombardment. An atomic layer of zirconium is thereby coated over thesurface of the needle 51 which, together with oxygen atoms from theresidual gas in the vacuum, act to reduce the work function at theemissive tip from 4.5 ev for pure tungsten to 2.8 ev. The reduced crosssectional dimension of the filament 50 at the vertex raises thetemperature of this region relative to the remaining length of thefilament and assures migration of the zirconium along the needle 51 inthe direction of the tip. The amount of zirconium material that need bedispensed is very little. A filament temperature of 1,800K in a mediumvacuum of about 10 mm. Hg keeps the cathode tip clean of adsorbed atoms.The described structure results in cathode lifetime that is extremelylong, e.g., on the order of 1,000 hours and greater. It

is noted that the optimum filament temperature is a function of pressureand for medium vacuum may exist in a range, typically, of 1,750K to1850K In FIG. 9 there are illustrated several field aided thermionicemission curves for both zirconium coated tungsten cathodes. and plaintungsten cathodes at different filament temperatures and for a fixedvacuum. The curves are plotted as emission current density in amperesper sq. cm. vs. electric field gradient in volts per cm. Curve Arepresents a pure tungsten cathode heated to a temperature of 2,000K.The curve is seen to cross the IOV/cm. field gradient line at a currentdensity of about 10 amperes per sq. cm. Curve B represents a zirconiumcoated tungsten cathode heated to a temperature of 1,500K, which is seento intersect the 10 V/cm. line at a current density of about 200 amperesper sq. cm. It is noted that although the filament temperature is lowerthan for curve A, the lowered work function of the zirconium coatedtungsten element appreciably increases the current emission. Curve Crepresents a pure tungsten cathode heated to a temperature of 2,600 K,which crosses the 10 V/cm. line at a current density of about 500amperes per sq. cm. It is seen that the elevated filament temperatureraises the current emission of the pure tungsten cathode above that ofcurves A and B. Curve D represents a pure tungsten cathode heated to atemperature of 3,000K, which provides an emission current density inexcess of 1,000 amperes per sq. cm. at the l0 V/cm line. Although highemission densities are achieved, the temperature of curves C and D arefound to be excessively high so as to drastically limit the lifetime ofthe cathode. Curve E represents a zirconium coated tungsten cathodeheated to a temperature of 1,800K, which is the type employed in thedescribed embodiment. It is seen that this curve attains an emissioncurrent density only slightly less than that of curve D but at a verymuch lower temperature. Thus, at 1,800K it is found that a high emissiondensity and high target current density is attained, and a stableoperation with long lifetime provided.

Referring again to FIG. 6, a pair of modulation coils 12 of standarddesign are mounted on opposing surfaces of a cylindrical sleeve 59 ofthe vacuum chamber 5, the sleeve being shown also in FIG. 7. Themodulation coils are employed to direct the beam along a single axis inthe X-Y plane, which is a plane transverse to the central axis Z of thebeam. Forward of the anode electrode 9 there is mounted a first magneticfocus lens in the form of focus coil 10A which is wound about thecircumference of the chamber 5 and produces a mag netic fieldpredominantly along the central axis of the beam. The coil 10A is per seof conventional type with its conductors enclosed by a magnetic ringcoil form. A gap 60 in the inner wall of the magnetic form locates thereference plane of the focus lens, which is at the middle of the gap atplane 61. The reference plane is used for spatially relating the focuscoils one to the other and to the object and image planes. The referenceplane is used for this purpose rather than the concept of principalplane because for these lenses the principal planes are not readilylocated. Forward of the first focus coil 10A is a second focus coil 108similar to the first, having a gap 62 in the coil form that places thereference plane of the second focus lens at plane 63. An astigmator coil64, of standard design, is wound about the chamber 5 for generating aproper axial magnetic field in the region of the the limiting aperture58. The astigmator coil is employed to compensate any astigmatism thatmay be produced by the focus coils A and 108. In addition, two pairs ofcentering coils 65, mounted on opposing surfaces of the vacuum chamberwall 66 in the vicinity of the lens plane 61 are provided for directingthe beam along two orthogonally disposed axes in the X-Y plane. Thecentering coils adjust the beam to pass through the center of the secondfocus lens. Two pair of deflection coils 11 mounted on opposing surfacesof the wall 66 forward of the second focus coil 10B deflect the beam intwo orthogonal directions in the X-Y plane.

Electron optics principles of the structure shown in FIGS. 6 and 7 willnow be discussed. Electrons emitted from the emissive surface at thecathode emission under the hemispherical electric field configurationwill generally be directed along diverging paths corresponding to radiiof the hemispherical electric field, said paths appearing to emanatefrom a point slightly behind the cathode emissive surface which may beconsidered as a virtual image of the cathode. Only a fraction of theemitted electrons are passed by the anode aperture 56, the passedelectrons being within about a 10 solid angle of the beam central axis.Of the electrons transmitted through the anode aperture 56 only a smallfraction, within a solid angle of about 1, are passed by the limitingaperture 58. The first focus coil 10A transposes the diverging beam intoa collimated beam. The second focus coil 10B transposes the parallelbeam into a converging beam which is focused on the surface of thestorage medium.

Spherical aberration of a focus lens, C,, is the most serious form oferror existing in electron optical systems, in general, with respect toproviding a sharply focused image. C, is primarily a function of lenspower, structural dimensions of the lens and accelerating voltage.Significantly, C, is inversely related to the lens power, or otherwiseconsidered, a direct function of the lens focal length. The presentconfiguration of the electron optical system very appreciably reducesspherical aberration of the system by minimizing the effective sphericalaberration C of each lens. C is defined as follows:

where a is the distance of the object plane or image plane to theprincipal plane of the lens, and f is the focal length of the lens.

Through the employment of a pair of focus lenses 10A and 108, thecathode emissive surface, corresponding approximately to the objectplane, may be located at about the focal point of the first focus lens10A. Thus, for each lens a f and C C,. This may be contrasted with usinga single focus lens for focusing the cathode object at the image planewhere to do so the object plane and image plane must be spaced at anappreciably greater distance than the focal length, so that a f and C CFrom the above consideration, spherical aberration of the system isreduced by increasing the power of focus coils 10A and 10B, withincertain limiting factors. With respect to coil 10A, the limiting factorsare principally the physical size and configuration of the anodestructure. With respect to coil 10B, the limiting factors are theplacement of the deflection coils 11 and the requirement for deflectingthe beam over a wide area. Where a reflective readout is employed, as inthe embodiment of FIG. 5, a further limiting factor is presented inpositioning of the electron detector in the region above the storagemedium.

In several exemplary operable embodiments of the electron opticsstructure, each of the focus coils 10A and 10B were identical and hadthe following specifications: the bore radius R 13/16 in., and the ratioS/D 3/13, where S is the gap width and D the bore diameter. The spacingof the cathode needle 51 to the plane 61, which is the dimension k inFIG. 6, was 1.5 in., the exact dimension having been dictated primarilyby the length of the anode electrode 9. The planes 61 and 63 were spacedapart by 3.5 in., dimension 1 in FIG. 6, which is sufficient toaccommodate placement of the cores but is not considered to be acritical dimension. The coil 10A was provided with ampere turns NI E 455at SKV, and N1 640 at 10 KV C, 4.85.

With the storage medium 1 spaced 1 in. from the plane 63, dimension 0 inFIG. 6, there were provided ampere turns N1 E 570 at an acceleratingvoltage of SKV, and N] E 810 at 10KV C,= 1.85. A spot size of 979 Adiameter was achieved. Power density at the focused spot was measured at6.64 X 10 watts per sq. cm. at 10 K V.

With the storage medium spaced 1.5 in. from the plane 63 N1 E 455 at5KV, and N1 E 640 at 1OKV C 4.85. A spot size of 1,058 A diameter wasachieved. Power density at the focused spot was measured at 5.69 X 10watts per sq. cm. at 5KV and 1.14 X 10 watts per sq. cm. at IOKV.

It may be appreciated that the angle of convergence of the beam at thestorage medium surface is an inverse function of the spacing of themedium 1 and the plane 63. The beam spot size is a function of a and Cand may be expressed as where d is the ideal spot size with zero error,and u is the half angle of the converging beam. In determining thespacing between the medium 1 and the plane 63, conflicting constraintsare present. C, decreases and 11 increases as the spacing is reduced.The selected spacing is optimized with respect to these properties, aswell as the requirement for scanning over a relatively large area.

The dimensions of the coils presented above are primarily for purposesof example and not intended to be limiting. Other size coils may andhave been employed, with the electrical parameters appropriatelymodified to provide operation in accordance with the present teachings.

During a writing operation the modulation coils l2 drive the beam alonga single axis in the X-Y plane, so as to shift the central axis of thebeam between a position in the center of the limiting aperture 58 and aposition offset from the center where the focused beam is partiallyblocked by the limiting apertured electrode 57. The beam is shifted as afunction of the modulation signal. With the central axis of the beam atthe center of the limiting aperture 56 the focused beam spot is ofmaximum current density and will readily machine the storage material.At the offset position, the focused beam spot current density is reducedsufficiently so that no machining of the storage material can occur.Thus as the beam is scanned along the data lines by the deflection coils11, the current intensity at the focused spot is modulated and datathereby written.

During a readout operation the modulation coils 12 are employed tofixedly bias the beam in the offset position so that the beam ispartially blocked by the limiting apertured electrode 57 for fixedlyreducing the current density of the focused spot. The beam of thereduced current density is scanned along the data lines by thedeflection coils 11 for providing readout of the stored data withouteffecting any physical change or destruction of said stored data.Alternatively, the anode voltage can be reduced during readout forreducing power density at the focused spot.

In FIG. 10 there is illustrated in a partially broken away perspectiveview a further embodiment of a storage system in accordance with theinvention employing a large area storage medium 71 composed of many datablocks 72 for providing a total storage capacity several orders ofmagnitude greater than that of embodiment of FIG. 1. A single data blockcorresponds to the storage area of the medium 1 in the embodiment ofFIG. 1. In the embodiment of FIG. 9, the total storage area of thestorage medium 71 is a plane surface about 210 X 210 square mm.,providing about 44,000 data blocks and a total storage capacity of 10bits. The data blocks are arranged in column and row configuration, onlyan exemplary number being shown in the partial plan view of FIG. 11.

The electron optics corresponds to the structure of FIGS. 6 and 7 andcomparable components are similarly identified but with an added primenotation. Accordingly, the electron emission system 3' and the electronoptical system 4' are identical to the previously considered embodiment.The input network 13', output network 17' and logic network 18 may besimilar to that of FIG. 1. Readout is preferably by means of areflective structure such as shown in FIG. employing the electrondetector 43 positioned above the storage surface. However, atransmissive readout similar to FIG. 1 may also be employed, requiring asuitable storage area electron detector structure for supporting thestorage medium.

The storage medium 71 is mounted on a movable substrate 73 forpositioning in both the X and Y direc tions. Movement of the substrate73 is provided by a pair of motor drive means 74 and 75 located outsideof the vacuum. Drive means 74 and 75 may include motors of conventionaltype which position the substrate 73 with an accuracy of i 1 mil. Avariable reactance stepping motor is suitable. Means 74 through a lineardrive shaft 76 drives the substrate 73 in the X direction, and means 75through a linear drive shaft 77 drives the substrate 73 in the Ydirection. The drive means 74 and 75 may each include a motiontranslation mechanism, such as a conventional ball screw-ball nutarrangement, for converting the motors rotational motion to the linearmotion of the drive shafts. A bellows such as shown at 78 provides avacuum seal around the drive shafts 76 and 77 while accommodating theirlinear motion.

Accordingly, in the operation of the system of FIG. 10, the motor drivemeans 74 and 75, under control of the logic network 18, provide indexingof individual data blocks 72 with respect to the electron beam structureand the electron beam. Upon a selected data block being indexed, theelectron beam may provide write and readout operations precisely asdescribed with respect to the previous embodiment of the invention.

The invention has been described with respect to a number of specificembodiments primarily for the purpose of clear and complete disclosure.It should be recognized, however, that numerous modifications may bemade to the disclosed structure by those skilled in the art which wouldnot exceed the present teaching. For example, the present electronoptical system has been employed to great advantage in combination witha novel electron emission system employing a zirconium coated orientedtungsten needle having a very low work function and capable of operatingclean in a medium vacuum so as to provide an extremely high emissiondensity and a long liftime. The combination of the described electronoptical system and the electron emission system produces at the target afocused beam spot of extremely small dimensions and high currentdensity. Conceivably, similar operation may be achieved by the presentelectron optical system in combination with other electron emissionsystems exhibit ing similar characteristics of high emission density,stability and low long lifetime in a medium vacuum. For example, ahafnium coated tungsten cathode or a lanthanum hexaboride cathode arebelieved to have the inherent properties for such operation, although todate are not known to have been suitably developed toward this use.

Further, within the concepts of present invention, the described storagesystems may employ for storing data a material whose physical state orproperties, other than or in addition to volume, are capable of beingchanged by a high power density beam, which change of state orproperties can be detected by a readout beam. In addition, the inventionis considered to embody use of a storage material capable of selectiveerasure by an electron beam.

It is also noted that the electron beam structure described herein mayhave useful application to other than information storage systems, forexample, to micromachining and micro-etching operations in the field ofmicroelectronics.

What is claimed as new and desired to be secured by Letters Patent inthe United States is:

1. An electron beam structure for forming a high resolution, highcurrent density electron beam, comprising:

a. an evacuated chamber,

b. cathode means within said chamber including a rigidly supportedcathode needle structure, the tip of which provides an extremely smalldimensioned emissive surface,

c. field means within said chamber for providing a generally radialelectric field centered about the cathode needle emissive tip with asufficiently high electric field gradient in the vicinity of theemissive tip for field emission, said cathode means and said field meansproducing a high density emission current formed into a beam having adivergent configuration,

d. a first focus lens for transposing the divergent configuration ofsaid beam into a collimated configuration, and I e. a second focus lensfor transposing the collimated configuration of said beam into aconvergent configuration, producing at an image plane an extremely smallspot at high current density.

2. An electron beam structure as in claim 1 wherein said cathode meansincludes filamentary heating means for aiding emission and means forcontinuously supplying a very thin coating to said tip of a materialthat reduces the work function at said tip.

3. An electron beam structure as in claim 2 wherein said emissive tip isin a plane at about the focal point of said first lens, and said imageplane is at about the focal point of said second lens so as to reduceeffective spherical aberration of said lenses.

4. An electron beam structure as in claim 3 in which said field meansincludes a grid electrode having an aperture through which said cathodeneedle protrudes, and an anode electrode positioned forward of saidemissive tip having an aperture coaxially related to said cathode needlethrough which the central portion of deflecting said beam over saidimage plane.

1. An electron beam structure for forming a high resolution, highcurrent density electron beam, comprising: a. an evacuated chamber, b.cathode means within said chamber including a rigidly supported cathodeneedle structure, the tip of which provides an extremely smalldimensioned emissive surface, c. field means within said chamber forproviding a generally radial electric field centered about the cathodeneedle emissive tip with a sufficiently high electric field gradient inthe vicinity of the emissive tip for field emission, said cathode meansand said field means producing a high density emission current formedinto a beam having a divergent configuration, d. a first focus lens fortransposing the divergent configuration of said beam into a collimatedconfiguration, and e. a second focus lens for transposing the collimatedconfiguration of said beam into a convergent configuration, producing atan image plane an extremely small spot at high current density.
 2. Anelectron beam structure as in claim 1 wherein said cathode meansincludes filamentary heating means for aiding emission and means forcontinuously supplying a very thin coating to said tip of a materialthat reduces the work function at said tip.
 3. An electron beamstructure as in claim 2 wherein said emissive tip is in a plane at aboutthe focal point of said first lens, and said image plane is at about thefocal point of said second lens so as to reduce effective sphericalaberration of said lenses.
 4. An electron beam structure as in claim 3in which said field means includes a grid electrode having an aperturethrough which said cathode needle protrudes, and an anode electrodepositioned forward of said emissive tip having an aperture coaxiallyrelated to said cathode needle through which the central portion of saidbeam is directed.
 5. An electron beam structure as in claim 4 whereinsaid first and second focus lenses each comprise a magnetic coil woundabout the circumference of said chamber.
 6. An electron beam structureas in claim 5 which further comprises deflection means positionedbetween the second magnetic focus coil and said image plane fordeflecting said beam over said image plane.